A storage apparatus is provided with a physical storage medium that is configured to store data and a controller that is configured to control a physical storage medium in general. The controller provides a data store space (a logical volume in general) to a computer (such as a host) that is coupled to a storage apparatus.
A storage apparatus enables an I/O processing to be executed at a high speed and can show a high fault tolerance to a failure of a physical storage medium by using a plurality of physical storage media with a RAID (Redundant Array of Independent (or Inexpensive) Disks) configuration.
A storage apparatus is equipped with an HDD (Hard Disk Drive) as a physical storage medium in general. In recent years however, a physical storage medium that is provided with a flash memory (hereafter referred to as an FM) such as an SSD (Solid State Drive) has attracted attention as a new physical storage medium as substitute for an HDD.
The SSD is provided with a merit of an extremely high speed of an I/O processing as compared with an HDD. However, there is an upper limit to the frequency of write of data for the SSD and a length of life of an SSD is shorter than that of an HDD disadvantageously. The disadvantages of the SSD will be described in the following.
In the case in which data of a flash memory (a flash memory of a NAND type typically) is tried to be rewritten, data cannot be over written on a physical region that has stored the data. In order to rewrite data to the data on the physical region, after an erasing processing is executed to data on the physical region in a unit of a block that is an erasing unit of a flash memory (hereafter referred to as a block erasing), it is necessary to write data on the physical region in which the block erasing has been executed.
However, the number of times of a block erasing (hereafter referred to as a frequency of erasing) for each block is limited because of a physical restriction of a flash memory. In the case in which a frequency of erasing of a block exceeds the limit, data cannot be stored into the block. In other words, a length of life of an SSD is when a frequency of erasing of all blocks that configure the SSD exceeds the upper limit.
A length of life of an SSD is lengthened by using a method called a wear leveling (hereafter referred to as a WL) for a general SSD. This is a technique for leveling a frequency of erasing between blocks and for suppressing only a specific block from being degraded by controlling a store location of data in such a manner that data that is updated with a low frequency is stored into a block that is provided with a more frequency of erasing and data that is updated with a high frequency is stored into a block that is provided with a less frequency of erasing.
In the case in which an SSD is adopted as a physical storage medium of a storage apparatus, a plurality of SSDs is mounted on the storage apparatus in general. In other words, even in the case in which only a specific block can be suppressed from being degraded, an imbalance occurs to loads between SSDs and a load is concentrated solely on a specific SSD in some cases. Patent Literature 1 discloses a method for implementing a long life of the entire of a storage apparatus by applying the WL to SSDs and by leveling an erasing frequency between a plurality of SSDs. A WL that is executed between physical storage media such as an SSD is referred to as an “inter-device WL” in the following.